1. Field of the Invention
This invention relates to a method of inspecting, testing, evaluating and repairing mill rolls used for flat rolled metal strip production in order to maximize the quality, production and cost efficiency potential of various types of rolls used in rolling flat strip products by detection, differentiation (classification) and correct application of methods for elimination of various defects found in mill rolls.
2. Background Information
Flat rolled strip production employs various types of steel and cast iron work rolls and backup rolls to reduce the thickness of steel slabs to the desired finished product thickness and width of flat rolled strip in coil form. The reduction in thickness employs high forces in both hot mills and cold rolling mills to elongate the steel bar and strip while delivering the desired physical and metallurgical properties to the strip product.
Flat rolling employs both continuous and semi-continuous rolling processes in hot mills and cold mills. Of critical importance is the thickness control, the shape and flatness, and the surface condition of the flat rolled strip. Variations in quality of these factors can result in processing cost increases, extra maintenance of equipment, production losses, and late deliveries of products to both downstream internal customers and external customers.
Various types of rolls are used in flat rolling, including cast iron, cast steel, high chrome iron, forged steel, tool steel and high speed steel. Rolls come in various sizes depending on the mill design ranging from 75 mm diameter to 2000 mm diameter and with a body length ranging from 1000 mm to 2500 mm. Rolls are commonly made from a variety of processes including: static casting, centrifugal spin casting, electro slag re-melt casting and continuous pour clad casting.
Roll performance is commonly evaluated by measurements including: total tonnage rolled, tonnage rolled per campaign, tonnage rolled per inch or mm of roll consumption, or specific roll force (force per unit width). Roll performance is affected by mill operation, rolling schedule, mill equipment condition, practices and procedures, product type and chemistry, roll inspection methods, roll maintenance procedures, roll use practices and roll inventory.
Rolls are highly susceptible to damage from a variety of failure modes, including: spalling, breakage, cracking, fatigue, wear, surface roughening, impression marks, bruising, hardness variation, or expression marks. Detection of defects is critical. However, current state of the art systems fail to allow differentiation of various types of cracks and the application of appropriate measures for the treatment of these cracks.
Various inventions about automatic inspection and testing of mill rolls have described ways and means for the use of eddy currents, ultrasonic sound and/or electro-magnetics to check the surface and interior of mill rolls normally used in the production of flat rolled steel strip and other flat rolled metallic products. Such inventions are described in U.S. Pat. Nos. 5,763,786; 4,495,587; and 3,939,404. To date, mill roll grinders and lathes use Computerized Numerical Control (CNC) programs to execute standard programs to repair and maintain the rolls. It is common for these programs to be supplied by the machine manufacturer and in many cases, it is difficult and time consuming to change the programs. These generic programs waste time and cause excessive material to be removed from the mill roll. Also, incorrect logic is used for new roll technology due to the lack of expert roll related knowledge available to the machine manufacturer. An improved method of controlling machine action will result in significant savings in metal consumption, machine and operator time and mill performance results. In addition, improved control will allow failure risk levels to be more carefully managed. Grind programs at the CNC level must be suitable to many various roll types. The program should react to many diverse inputs and accommodate changes as demanded by the quality level of the product being manufactured.
The invention provides a method for inspecting a mill roll used for producing flat rolled metal strip and for disposition of a mill roll of predetermined type and in service on a predetermined mill stand as a function of any defects detected in said mill roll, the method comprising the steps of:
a) applying a nondestructive inspection system to generate variable amplitude output voltage signals corresponding to changes in physical properties along a reference direction for at least a portion of the mill roll, any said changes in physical properties corresponding to a mill roll defect;
b) defining an output voltage signal pattern from said variable amplitude output voltage signals;
c) classifying said mill roll defect in accordance with predetermined patterns of output voltage signals;
d) selecting a threshold signal value corresponding to said classified mill roll defect for the type of mill roll being inspected on said mill stand;
e) calculating a difference between a maximum peak height for the output voltage signals and said threshold signal value; and
f) defining corrective action for disposition of the mill roll in accordance with said calculated difference.
The invention allows automatic decision making for roll maintenance practices to be employed in order to attain the highest performance possible, without incurring risk of roll failure and without incurring risk of lost mill operating time or quality defects when using said work rolls or backup rolls.
The invention also provides a system for inspecting mill rolls used in the production of flat rolled metal strip and for defining corrective action for disposition of mill rolls, the system comprising:
a) data input means for receiving data defining a roll history and mill history associated with a roll being inspected;
b) a database of threshold signal values each corresponding to an acceptable output voltage signal for a roll having a predetermined roll history and mill history and having a predetermined type of mill roll defect;
c) a nondestructive inspection system for generating variable amplitude output voltage signals corresponding to changes in physical properties associated with mill roll defects;
d) signal processing means for receiving said variable amplitude output voltage signals and defining a voltage signal pattern;
e) signal classification means for classifying said voltage signal pattern in accordance with predetermined patterns of output voltage signals associated with predetermined types of mill roll defects each having a predetermined threshold signal value;
f) computation means to calculate a difference in a maximum peak height between the output voltage signals characterizing said voltage signal pattern and said predetermined threshold signal value and to define corrective action for disposition of the mill roll; and
g) transfer means for conveying information about said corrective action.
Various roll grinder process measurements are taken automatically by a Grinder CNC Control Program, including caliper measurements of the roll diameter, roll temperature, shape, runout and eccentricity. Data values are transferred automatically to a database and stored. The CNC control system also displays values in chart output format on a CRT (cathode ray tube) display. An automatic grind program selection module in the computer control system chooses the correct CNC grind program to achieve the optimum target grind level for the specified roll type, mill stand and rolling schedule type.
Grind program selection logic is developed offline and is based on roll performance management strategies developed in the operating plant. Key functions include the target grind levels for each roll type, the tolerance levels and risk levels permitted by the rolling operation, and the optimum use strategy employed at the rolling operation.
In addition to grinder process measurements, automatic eddy current and ultrasonic testing equipment measurements are transferred automatically to a database and stored. The database also contains a table of threshold values for each roll type, mill stand, and rolling schedule. A second computer module compares the output values of the grinding process and the output values of the eddy current and ultrasonic test equipment to the threshold values and an automatic decision is made to return to a grind program selection module or to enter a finish grind program module.
A third computer module evaluates the output values of the eddy current and ultrasonic systems and through a process of logical steps classifies the various defect types which may exist within the roll surface, shell or core. These classified types, along with their severity ratings are passed to the threshold comparison module that then allows the correct grind program selection to take place.
The ability to automatically detect defects within the roll surface, shell and core using automatic detection equipment is valuable information. Utilizing the automatic defect recognition software elevates the value of such information considerably. A model predicts the defect type based on statistical information and standard values that have been established by review and study of many and various types of rolls. Standard values are developed by resident experts in roll technology, roll testing and process experts off line.
Threshold values are established offline based on many roll condition observations in the operating plant and are formulated into a useful table. The collection and organization of these threshold values is the key to the ability of the grind program selection module to function for performance optimization.
A graphical tabulation of threshold signal values is provided, the threshold signal values each corresponding to an acceptable output voltage signal for a roll having a predetermined roll history and mill history and having a predetermined type of mill roll defect. The threshold signal values are preferably displayed as a bar chart with the height of a bar associated with a first axis corresponding to output voltage signals generated by a nondestructive inspection system and a second axis corresponding to a depth of material to be removed from a mill roll.
In accordance with another aspect of the invention, a roll mill insert comprising a plate for mounting on a mill roll is provided, the plate having defects formed therein for generating output voltage signals of predetermined amplitude to calibrate a nondestructive inspection system.